This invention relates to a compression-ignition internal combustion engine, such as a diesel engine, equipped with an exhaust gas recirculation (EGR) system and more particularly relates to an intake system for making up for a deficiency in an intake air amount when EGR is cut off in a definite range of engine operating conditions.
In a conventional EGR system in a compression-ignition internal combustion engine, such as a diesel engine, a throttle valve is disposed in an intake passage to obtain a negative pressure downstream of the throttle valve and a portion of exhaust recirculation gases is sucked by the negative pressure for recirculation. The quantity of EGR is controlled in accordance with the magnitude of the negative pressure which is determined by the opening degree of the throttle valve.
In such an EGR system, exhaust gases are recirculated in accordance with the opening of the throttle valve without exception. However there are some engine operating conditions where EGR is not needed, for example, when starting a cold engine or during acceleration in which a sufficient supply of fresh air is demanded by the engine. To perform EGR when starting an engine leads to a difficulty in starting the engine and EGR during acceleration incurs a danger of production of smoke.
To meet these problems, an EGR system shown in FIG. 1 can be devised. In FIG. 1, an EGR cut-off valve 1 of a pressure responsive type is disposed in an EGR passage 2 connecting an exhaust passage 3 with an induction passage 4 at a portion downstream of a throttle valve 5. Also a by-pass cut-off valve 6 of a pressure responsive type is disposed in a bypass passage 7 which is arranged to detour the throttle valve 5 in the induction passage. These two cut-off valves 1 and 6 are arranged to cooperate in such a manner that the by-pass cut-off valve 6 is open when the EGR cut-off valve 1 is closed so as to increase an intake air amount immediately after EGR is cut off.
In such a system, EGR is cut off by closing the EGR valve 1, for example, when starting the engine, when the engine is cold or during high speed or acceleration, and at the same time a deficiency in intake air amount caused by the cut-off of EGR is compensated for by opening the by-pass cut-off valve 6. For doing this, there is provided a three-way solenoid control valve 8 which is actuated by an output signal from a device 9 for detecting engine operating condition and which is arranged to supply through a passage 10 a negative pressure from a vacuum source 11 or an atmospheric pressure to the EGR valve or the bypass valve 6 alternatively. The detecting device 9 comprises a plurality of sensors such as a speed sensor for detecting the rotational speed of the engine, a sensor for detecting the load of the engine and a sensor for detecting a starting of the engine.
In this EGR system, however, it is necessary to install two valves, that is, the EGR cut-off valve 1 in the EGR passage 2 and the bypass cut-off valve 6 in the bypass passage 7 and this leads to an increase in the number of parts to constitute the system and hence to an increase in cost and a complicated design having an intricate cooperative mechanism.